1. Field of the Invention
The present invention relates to a lighting device and a display device and a liquid display device in which the lighting device is used and, more particularly, to a technique effectively applied to a lighting device that realizes a planar light source by using a light guide and a display device, in particular, a liquid crystal display device in which the lighting device is used as a backlight.
2. Description of the Related Art
A display device is a medium that visually transmits information to humans. In the modern advanced information society, the display device is an important entity for the humans and the society. In particular, the performance of a liquid crystal display device has been remarkably improved in recent years. The liquid crystal display device is adopted as a display device mounted on a cellular phone terminal, a display device connected to a personal computer and used, and a display device of a large-screen television and the like. In general, the liquid crystal display device includes a liquid crystal display panel and a backlight (a lighting device) in the back of the liquid crystal display panel and configured to irradiate light on the liquid crystal display panel.
The liquid crystal display device including the backlight (hereinafter simply referred to as “liquid crystal display device”) adjusts, independently in respective pixels, an amount of transmission of light, which is emitted from the backlight, through the liquid crystal display panel to display a video and an image. As the liquid crystal display panel, a liquid crystal display panel that includes a pair of polarizers and a liquid crystal layer arranged between the pair of polarizers and controls a polarization state of light passing through the liquid crystal layer to perform display is desirable because a video with a high contrast ratio can be obtained at a relatively low driving voltage. As a display system of such a liquid crystal display panel, for example, a TN (Twisted Nematic) system, an STN (Super Twisted Nematic) system, an ECB (Electrical Controlled Birefringence) system, an IPS (In-Plane Switching) system, and a VA (Vertical Aligned) system are known. In all the display systems, the liquid crystal display panel usually includes a pair of substrates, a liquid crystal layer held between the substrates, and a pair of polarizers arranged across the liquid crystal layer and the pair of substrates. The liquid crystal display panel changes a polarization state of light entering the liquid crystal layer to control a transmitted light amount of the light in respective pixels and display a video and an image.
The polarizers used in the liquid crystal display panel have a function of absorbing predetermined linearly polarized light components and transmitting linearly polarized light having a vibration plane orthogonal to a vibration plane of the predetermined linearly polarized light components. Therefore, when light from the backlight (hereinafter referred to as backlight light) irradiated on the liquid crystal display panel is unpolarized light, the polarizers provided in the liquid crystal display panel absorb at least 50% of the backlight light. In other words, in the liquid crystal display device, when the backlight light is the unpolarized light, about a half of the backlight light is absorbed by the polarizers and lost. Therefore, in realizing a brighter image or a liquid crystal display device that consumes low electric power, it is important to reduce a ratio of the backlight light absorbed by the polarizers provided in the liquid crystal display panel and improve utilization efficiency of the backlight light.
As the backlight of the liquid crystal display device, for example, there are an edge light type (a light guide body type), a direct type (a reflector type), and a planar light source type. These types are properly used according to an application or the like of the liquid crystal display device. The backlight of the edge light type is easily reduced in thickness, reduced in weight, and reduced in power consumption compared with the other types and is often used in, for example, a liquid crystal display device mounted on a portable electronic apparatus. The backlight of the edge light type includes a liner or point-like light source, alight guide that spreads light from the light source in a planar shape and emits the light to the liquid crystal display panel side, optical sheets such as a prism sheet and a diffusion sheet, and a reflection sheet.
Lights emitted from the light guide in the backlight of the edge light type (hereinafter simply referred to as “backlight”) generally have a maximum (a peak) of luminance or luminous intensity in a direction tilting 50 degrees to 80 degrees with respect to a perpendicular (normal) direction of an light emission surface in the light guide. Therefore, the optical sheets such as the diffusion sheet and the prism sheet are arranged between the light guide and the liquid crystal display panel to adjust a traveling direction of the lights emitted from the light guide and set a direction in which the luminance or the luminous intensity is maximized in a front direction (i.e., the perpendicular direction of the light emission surface of the light guide) or a direction near the front direction.
As a general configuration of the backlight, the reflection sheet is arranged on the rear surface side of the light guide and the diffusion sheet and two prism sheets are arranged in this order on a planar light emission surface side, which is the front surface, of the light guide. In this case, the diffusion sheet acts to diffuse the lights emitted from the light guide and expand a traveling range of the light. Further, the diffusion sheet also acts to bring a peak angle (an angle at which the luminance or the luminous intensity is maximized) of the lights emitted from the light guide close to the front direction (the perpendicular direction of the light emission surface in the light guide). On the other hand, the prism sheets are light-transmitting optical members in which plural prisms are continuously arranged on a sheet-like substrate. In general, the cross-sectional shape of the prisms is a triangular shape having a vertex angle of 90 degrees. In general, such two prism sheets are arranged such that ridgelines of the prisms of the prism sheets are orthogonal to each other.
Emitted light of such a backlight is substantially unpolarized light. Therefore, at least 50% of the light emitted from the backlight is absorbed by the polarizers provided in the liquid crystal display panel and lost. In order to suppress the absorption loss in the polarizers of the light emitted from the backlight, two methods are mainly proposed. One is a method of arranging a reflective polarizer between the backlight and the liquid crystal display panel and the other is a method of using a backlight that emits light with a high ratio of the predetermined linearly polarized light components.
The reflective polarizer acts as explained below. Among lights emitted from the backlight and traveling to the liquid crystal display panel, polarized light components to be absorbed by the polarizers and lost are reflected on the reflective polarizer before being absorbed by the polarizers. The light reflected on the reflective polarizer is reflected on the backlight as well and travels to the liquid crystal display panel as well. At this point, a polarization state of the light reflected on the backlight changes. Therefore, a part of the light traveling to the liquid crystal display panel as well passes through the reflective polarizer and the polarizers to be used for display. In other words, the absorption loss in the polarizers decreases because the liquid crystal display device includes the reflective polarizer. Therefore, the luminance of the liquid crystal display device can be increased.
On the other hand, as the backlight that emits light with a high ratio of the predetermined linearly polarized light components, for example, there is a backlight disclosed in JP 2010-262813 A. The backlight disclosed in JP 2010-262813 A makes use of the fact that, among lights emitted from the light guide, in light emitted at a tilt angle of about 60 degrees to 80 degrees with respect to the perpendicular direction of the light emission surface of the light guide, the number of p-polarized light components is larger than the number of s-polarized light components. On the light emission surface side of the light guide, a prism sheet configured to preferentially guide, to the front direction, light having a large amount of p-polarized light components emitted from the light guide is used. When the prism sheet is used, the backlight having a large light amount of the predetermined linearly polarized light components is realized by also making use of polarization dependency of transmittance on an interface between the prism sheet and the air.